1. Field of the Invention
This invention relates to gyrotron devices for generating electromagnetic waves, and more particularly, to gyrotron devices applied to electron cyclotron resonance heating, i.e., for heating plasma in nuclear fusion reactors with the electromagnetic waves.
2. Discussion of Background
FIG. 1 shows a gyrotron device of the above-noted type, whose construction includes an electron gun 1 for emitting an electron beam in the direction of arrow Z, a magnetic coil 2 for giving a cyclotron movement to electrons in the electron beam emitted from the electron gun 1, a cavity resonator 3 for resonating the electromagnetic wave generated from the electron beam, and an output section 5 for transmitting the electromagnetic wave through an output window 4. The inner diameter of the cavity resonator 3, generally, becomes small in proportion to the wavelength of the electromagnetic wave.
When an electromagnetic wave with a frequency higher than 100 GHz and a power level of megawatts is generated by using the gyrotron device with the above-mentioned cavity resonator 3, the resonating frequency of the electromagnetic wave in the cavity resonator 3 is so high that the resonator 3 cannot have an inner diameter large enough to reduce Joule heat to a tolerable level. As a result, the ohmically heated inner wall of the cavity resonator 3 inevitably receives extremely high heat (&gt;1 KW/cm.sup.2). This gyrotron device, therefore, cannot practically supply high power continuous or long pulse electromagnetic waves having a frequency higher than 100 GHz. A complex system having plural gyrotron devices must then be constructed to achieve the electron cyclotron resonance heating of fusion plasma.
In the above-mentioned gyrotron device, the electromagnetic wave is transmitted through the output window 4 in an optional direction. This requires the use of a waveguide for transmitting the wave to the desired place. When the electromagnetic wave is transmitted in this manner through the waveguide, its energy is gradually decreased due to ohmic losses, i.e., the transmission efficiency of the electromagnetic wave is reduced. In addition, if the electromagnetic wave is transmitted through the waveguide, it is difficult to focus the wave power onto a desired object. This is another reason why the above-mentioned gyrotron device is unfavorable for heating plasma in a nuclear fusion reactor.
Another type of gyrotron device is known which uses a Fabry-Perot resonator. This device is called a "quasi-optical gyrotron". The axis of its resonator is perpendicular to that of magnet coils which generate a magnetic field to guide an electron beam emitted by an electron gun. The device is thus non-axisymmetric, which requires complicated positional adjustments of mirrors, the electron gun, magnet coils, and the like, which inevitably raises the cost of manufacturing the quasi-optical gyrotron.